U.S. patent number 9,781,747 [Application Number 14/763,473] was granted by the patent office on 2017-10-03 for method and apparatus for transmitting random access channel designed for transmission in high carrier frequency in a wireless communication system.
This patent grant is currently assigned to LG ELECTRONICS INC.. The grantee listed for this patent is LG ELECTRONICS INC.. Invention is credited to Jaehoon Chung, Jinmin Kim, Kitae Kim, Hyunsoo Ko.
United States Patent |
9,781,747 |
Kim , et al. |
October 3, 2017 |
Method and apparatus for transmitting random access channel
designed for transmission in high carrier frequency in a wireless
communication system
Abstract
A method for transmitting a random access preamble at a user
equipment in a wireless communication system is disclosed. The
method includes determining a random access sequence transmission
period using a random access sequence, configuring a random access
preamble by inserting a cyclic prefix at a front of the random
access sequence transmission period and a guard time at an end of
the random access sequence transmission period, and transmitting
the random access preamble on a random access channel to a base
station. The random access preamble is configured by repeating the
random access sequence a predetermined number of times, if the
length of the random access sequence is smaller than the random
access sequence transmission period.
Inventors: |
Kim; Kitae (Anyang-si,
KR), Kim; Jinmin (Anyang-si, KR), Ko;
Hyunsoo (Anyang-si, KR), Chung; Jaehoon
(Anyang-si, KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
LG ELECTRONICS INC. |
Seoul |
N/A |
KR |
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Assignee: |
LG ELECTRONICS INC. (Seoul,
KR)
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Family
ID: |
51262521 |
Appl.
No.: |
14/763,473 |
Filed: |
August 20, 2013 |
PCT
Filed: |
August 20, 2013 |
PCT No.: |
PCT/KR2013/007452 |
371(c)(1),(2),(4) Date: |
July 24, 2015 |
PCT
Pub. No.: |
WO2014/119832 |
PCT
Pub. Date: |
August 07, 2014 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150373743 A1 |
Dec 24, 2015 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61757720 |
Jan 29, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L
27/26132 (20210101); H04L 5/0048 (20130101); H04L
27/2613 (20130101); H04W 74/0833 (20130101) |
Current International
Class: |
H04W
74/08 (20090101); H04L 5/00 (20060101); H04L
27/26 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
PCT International Application No. PCT/KR2013/007452, Written
Opinion of the International Searching Authority dated Dec. 24,
2013, 9 pages. cited by applicant.
|
Primary Examiner: Orgad; Edan
Assistant Examiner: Shah; Saumit
Attorney, Agent or Firm: Lee Hong Degerman Kang Waimey
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is the National Stage filing under 35 U.S.C. 371
of International Application No. PCT/KR2013/007452, filed on Aug.
20, 2013, which claims the benefit of U.S. Provisional Application
No. 61/757,720, filed on Jan. 29, 2013, the contents of which are
all hereby incorporated by reference herein in their entirety.
Claims
The invention claimed is:
1. A method for transmitting a random access preamble at a user
equipment in a wireless communication system, the method
comprising: determining a random access sequence transmission
period using a random access sequence; configuring a random access
preamble by inserting a cyclic prefix at a front of the random
access sequence transmission period and a guard time at an end of
the random access sequence transmission period; and transmitting
the random access preamble on a random access channel to a base
station, wherein configuring the random access preamble comprises
configuring the random access preamble by repeating the random
access sequence a predetermined number of times along a frequency
axis, if the length of the random access sequence is smaller than
the random access sequence transmission period, wherein the random
access sequence x.sub.m repeated the predetermined number of times
is expressed as [Equation A],
.times..times..ltoreq.<.times..times. ##EQU00017## and wherein
x.sub.p is a random access sequence of length N.sub.SEQ, and R is
the predetermined number.
2. The method according to claim 1, wherein the predetermined
number is a quotient of dividing the random access sequence
transmission period by the length of the random access
sequence.
3. The method according to claim 1, wherein a subcarrier spacing of
the random access channel is increased by a multiple of the
predetermined number.
4. A user equipment in a wireless communication system, the user
equipment comprising: a processor for determining a random access
sequence transmission period using a random access sequence and
configuring a random access preamble by inserting a cyclic prefix
at a front of the random access sequence transmission period and a
guard time at an end of the random access sequence transmission
period; and a transmission module for transmitting the random
access preamble on a random access channel to a base station,
wherein the processor configures the random access preamble by
repeating the random access sequence a predetermined number of
times along a frequency axis, if the length of the random access
sequence is smaller than the random access sequence transmission
period, wherein the random access sequence x.sub.m repeated the
predetermined number of times is expressed as [Equation A],
.times..times..ltoreq.<.times..times. ##EQU00018## and wherein
x.sub.p is a random access sequence of length N.sub.SEQ and R is
the predetermined number.
5. The user equipment according to claim 4, wherein the
predetermined number is a quotient of dividing the random access
sequence transmission period by the length of the random access
sequence.
6. The user equipment according to claim 4, wherein a subcarrier
spacing of the random access channel is increased by a multiple of
the predetermined number.
Description
TECHNICAL FIELD
The present invention relates to a wireless communication system,
and more particularly, to a method and apparatus for transmitting a
random access channel designed for transmission in a high carrier
frequency in a wireless communication system.
BACKGROUND ART
A brief description will be given of a 3rd Generation Partnership
Project Long Term Evolution (3GPP LTE) system as an example of a
wireless communication system to which the present invention can be
applied.
FIG. 1 illustrates a configuration of an Evolved Universal Mobile
Telecommunications System (E-UMTS) network as an exemplary wireless
communication system. The E-UMTS system is an evolution of the
legacy UMTS system and the 3GPP is working on the basics of E-UMTS
standardization. E-UMTS is also called an LTE system. For details
of the technical specifications of UMTS and E-UMTS, refer to
Release 7 and Release 8 of "3rd Generation Partnership Project;
Technical Specification Group Radio Access Network",
respectively.
Referring to FIG. 1, the E-UMTS system includes a User Equipment
(UE), an evolved Node B (eNode B or eNB), and an Access Gateway
(AG) which is located at an end of an Evolved UMTS Terrestrial
Radio Access Network (E-UTRAN) and connected to an external
network. The eNB may transmit multiple data streams simultaneously,
for broadcast service, multicast service, and/or unicast
service.
A single eNB manages one or more cells. A cell is set to operate in
one of the bandwidths of 1.4, 3, 5, 10, 15 and 20 Mhz and provides
Downlink (DL) or Uplink (UL) transmission service to a plurality of
UEs in the bandwidth. Different cells may be configured so as to
provide different bandwidths. An eNB controls data transmission and
reception to and from a plurality of UEs. Regarding DL data, the
eNB notifies a particular UE of a time-frequency area in which the
DL data is supposed to be transmitted, a coding scheme, a data
size, Hybrid Automatic Repeat reQuest (HARM) information, etc. by
transmitting DL scheduling information to the UE. Regarding UL
data, the eNB notifies a particular UE of a time-frequency area in
which the UE can transmit data, a coding scheme, a data size, HARQ
information, etc. by transmitting UL scheduling information to the
UE. An interface for transmitting user traffic or control traffic
may be defined between eNBs. A Core Network (CN) may include an AG
and a network node for user registration of UEs. The AG manages the
mobility of UEs on a Tracking Area (TA) basis. A TA includes a
plurality of cells.
While the development stage of wireless communication technology
has reached LTE based on Wideband Code Division Multiple Access
(WCDMA), the demands and expectation of users and service providers
are increasing. Considering that other radio access technologies
are under development, a new technological evolution is required to
achieve future competitiveness. Specifically, cost reduction per
bit, increased service availability, flexible use of frequency
bands, a simplified structure, an open interface, appropriate power
consumption of UEs, etc. are required.
DISCLOSURE
Technical Problem
An object of the present invention devised to solve the problem
lies on a method and apparatus for transmitting a random access
channel designed for transmission in a high carrier frequency in a
wireless communication system.
Technical Solution
The object of the present invention can be achieved by providing a
method for transmitting a random access preamble at a user
equipment in a wireless communication system is disclosed. The
method includes determining a random access sequence transmission
period using a random access sequence, configuring a random access
preamble by inserting a cyclic prefix at a front of the random
access sequence transmission period and a guard time at an end of
the random access sequence transmission period, and transmitting
the random access preamble on a random access channel to a base
station. The random access preamble is configured by repeating the
random access sequence a predetermined number of times, if the
length of the random access sequence is smaller than the random
access sequence transmission period.
In another aspect of the present invention, provided herein is a
user equipment in a wireless communication system, including a
processor for determining a random access sequence transmission
period using a random access sequence and configuring a random
access preamble by inserting a cyclic prefix at a front of the
random access sequence transmission period and a guard time at an
end of the random access sequence transmission period, and a
transmission module for transmitting the random access preamble on
a random access channel to a base station. The processor configures
the random access preamble by repeating the random access sequence
a predetermined number of times, if the length of the random access
sequence is smaller than the random access sequence transmission
period.
If the length of the random access sequence is smaller than the
random access sequence transmission period, the random access
preamble may be configured by repeating the random access sequence
the predetermined number of times along a time axis or along a
frequency axis.
The predetermined number may be a quotient of dividing the random
access sequence transmission period by the length of the random
access sequence.
If the random access preamble is configured by repeating the random
access sequence the predetermined number of times along the
frequency axis, a subcarrier spacing of the random access channel
may be increased by a multiple of the predetermined number.
The random access sequence x.sub.m repeated the predetermined
number of times may be expressed as [Equation A],
.times..times..ltoreq.<.times..times. ##EQU00001##
where x.sub.p is a random access sequence of length N.sub.SEQ and R
is the predetermined number.
Advantageous Effects
According to the embodiments of the present invention, a User
Equipment (UE) can efficiently transmit a random access channel in
a high carrier frequency in a wireless communication system.
It will be appreciated by persons skilled in the art that the
effects that can be achieved with the present invention are not
limited to what has been particularly described hereinabove and
other advantages of the present invention will be more clearly
understood from the following detailed description taken in
conjunction with the accompanying drawings.
DESCRIPTION OF DRAWINGS
The accompanying drawings, which are included to provide a further
understanding of the invention, illustrate embodiments of the
invention and together with the description serve to explain the
principle of the invention.
In the drawings:
FIG. 1 illustrates a configuration of an Evolved Universal Mobile
Telecommunications System (E-UMTS) network as an example of a
wireless communication system;
FIG. 2 illustrates a control-plane protocol stack and a user-plane
protocol stack in a radio interface protocol architecture
conforming to a 3rd Generation Partnership Project (3GPP) radio
access network standard between a User Equipment (UE) and an
Evolved UMTS Terrestrial Radio Access Network (E-UTRAN);
FIG. 3 illustrates physical channels and a general signal
transmission method using the physical channels in a 3GPP
system;
FIG. 4 illustrates a structure of a downlink radio frame in a Long
Term Evolution (LTE) system;
FIG. 5 illustrates a structure of an uplink subframe in the LTE
system;
FIG. 6 illustrates the concept of a small cell, which is expected
to be introduced to the LTE system;
FIG. 7 illustrates a structure of a Random Access Channel (RACH)
preamble;
FIG. 8 illustrates an exemplary setting of a Cyclic Prefix (CP) and
a transmission period for an RACH preamble to transmit the RACH
preamble in a high carrier frequency;
FIG. 9 illustrates another exemplary setting of a CP and a
transmission period for an RACH preamble to transmit the RACH
preamble in a high carrier frequency;
FIG. 10 is a graph illustrating RACH sequence length versus service
coverage in the LTE system;
FIG. 11 is a graph illustrating RACH sequence length versus service
coverage in a high carrier frequency system;
FIG. 12 illustrates a relationship between an RACH subcarrier
spacing and an RACH sequence length;
FIG. 13 illustrates an exemplary setting of the length of an RACH
sequence that is transmitted twice repeatedly during a Transmission
Time Interval (TTI) according to an embodiment of the present
invention;
FIG. 14 illustrates an exemplary allocation of an RACH sequence in
the frequency domain, for one-period transmission during a TTI
according to an embodiment of the present invention; and
FIG. 15 is a block diagram of a communication apparatus according
to an embodiment of the present invention.
BEST MODE
The configuration, operation, and other features of the present
invention will readily be understood with embodiments of the
present invention described with reference to the attached
drawings. Embodiments of the present invention as set forth herein
are examples in which the technical features of the present
invention are applied to a 3rd Generation Partnership Project
(3GPP) system.
While embodiments of the present invention are described in the
context of Long Term Evolution (LTE) and LTE-Advanced (LTE-A)
systems, they are purely exemplary. Therefore, the embodiments of
the present invention are applicable to any other communication
system as long as the above definitions are valid for the
communication system. In addition, while the embodiments of the
present invention are described in the context of Frequency
Division Duplexing (FDD), they are also readily applicable to
Half-FDD (H-FDD) or Time Division Duplexing (TDD) with some
modifications.
FIG. 2 illustrates control-plane and user-plane protocol stacks in
a radio interface protocol architecture conforming to a 3GPP
wireless access network standard between a User Equipment (UE) and
an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). The
control plane is a path in which the UE and the E-UTRAN transmit
control messages to manage calls, and the user plane is a path in
which data generated from an application layer, for example, voice
data or Internet packet data is transmitted.
A PHYsical (PHY) layer at Layer 1 (L1) provides information
transfer service to its higher layer, a Medium Access Control (MAC)
layer. The PHY layer is connected to the MAC layer via transport
channels. The transport channels deliver data between the MAC layer
and the PHY layer. Data is transmitted on physical channels between
the PHY layers of a transmitter and a receiver. The physical
channels use time and frequency as radio resources. Specifically,
the physical channels are modulated in Orthogonal Frequency
Division Multiple Access (OFDMA) for downlink and in Single Carrier
Frequency Division Multiple Access (SC-FDMA) for uplink.
The MAC layer at Layer 2 (L2) provides service to its higher layer,
a Radio Link Control (RLC) layer via logical channels. The RLC
layer at L2 supports reliable data transmission. RLC functionality
may be implemented in a function block of the MAC layer. A Packet
Data Convergence Protocol (PDCP) layer at L2 performs header
compression to reduce the amount of unnecessary control information
and thus efficiently transmit Internet Protocol (IP) packets such
as IP version 4 (IPv4) or IP version 6 (IPv6) packets via an air
interface having a narrow bandwidth.
A Radio Resource Control (RRC) layer at the lowest part of Layer 3
(or L3) is defined only on the control plane. The RRC layer
controls logical channels, transport channels, and physical
channels in relation to configuration, reconfiguration, and release
of Radio Bearers (RBs). An RB refers to a service provided at L2,
for data transmission between the UE and the E-UTRAN. For this
purpose, the RRC layers of the UE and the E-UTRAN exchange RRC
messages with each other. If an RRC connection is established
between the UE and the E-UTRAN, the UE is in RRC Connected mode and
otherwise, the UE is in RRC Idle mode. A Non-Access Stratum (NAS)
layer above the RRC layer performs functions including session
management and mobility management.
A cell covered by an eNB is set to one of the bandwidths of 1.4, 3,
5, 10, 15, and 20 MHz and provides downlink or uplink transmission
service in the bandwidth to a plurality of UEs. Different cells may
be set to provide different bandwidths.
Downlink transport channels used to deliver data from the E-UTRAN
to UEs include a Broadcast Channel (BCH) carrying system
information, a Paging Channel (PCH) carrying a paging message, and
a Shared Channel (SCH) carrying user traffic or a control message.
Downlink multicast traffic or control messages or downlink
broadcast traffic or control messages may be transmitted on a
downlink SCH or a separately defined downlink Multicast Channel
(MCH). Uplink transport channels used to deliver data from a UE to
the E-UTRAN include a Random Access Channel (RACH) carrying an
initial control message and an uplink SCH carrying user traffic or
a control message. Logical channels that are defined above
transport channels and mapped to the transport channels include a
Broadcast Control Channel (BCCH), a Paging Control Channel (PCCH),
a Common Control Channel (CCCH), a Multicast Control Channel
(MCCH), a Multicast Traffic Channel (MTCH), etc.
FIG. 3 illustrates physical channels and a general method for
transmitting signals on the physical channels in the 3GPP
system.
Referring to FIG. 3, when a UE is powered on or enters a new cell,
the UE performs initial cell search (S301). The initial cell search
involves acquisition of synchronization to an eNB. Specifically,
the UE synchronizes its timing to the eNB and acquires a cell
Identifier (ID) and other information by receiving a Primary
Synchronization Channel (P-SCH) and a Secondary Synchronization
Channel (S-SCH) from the eNB. Then the UE may acquire information
broadcast in the cell by receiving a Physical Broadcast Channel
(PBCH) from the eNB. During the initial cell search, the UE may
monitor a downlink channel state by receiving a DownLink Reference
Signal (DL RS).
After the initial cell search, the UE may acquire detailed system
information by receiving a Physical Downlink Control Channel
(PDCCH) and receiving a Physical Downlink Shared Channel (PDSCH)
based on information included in the PDCCH (S302).
If the UE initially accesses the eNB or has no radio resources for
signal transmission to the eNB, the UE may perform a random access
procedure with the eNB (S303 to S306). In the random access
procedure, the UE may transmit a predetermined sequence as a
preamble on a Physical Random Access Channel (PRACH) (S303 and
S305) and may receive a response message to the preamble on a PDCCH
and a PDSCH associated with the PDCCH (S304 and S306). In case of a
contention-based RACH, the UE may additionally perform a contention
resolution procedure.
After the above procedure, the UE may receive a PDCCH and/or a
PDSCH from the eNB (S307) and transmit a Physical Uplink Shared
Channel (PUSCH) and/or a Physical Uplink Control Channel (PUCCH) to
the eNB (S308), which is a general downlink and uplink signal
transmission procedure. Particularly, the UE receives Downlink
Control Information (DCI) on a PDCCH. Herein, the DCI includes
control information such as resource allocation information for the
UE. Different DCI formats are defined according to different usages
of DCI.
Control information that the UE transmits to the eNB on the uplink
or receives from the eNB on the downlink includes a DL/UL
ACKnowledgment/Negative ACKnowledgment (ACK/NACK) signal, a Channel
Quality Indicator (CQI), a Precoding Matrix Index (PMI), a Rank
Indicator (RI), etc. In the 3GPP LTE system, the UE may transmit
control information such as a CQI, a PMI, an RI, etc. on a PUSCH
and/or a PUCCH.
FIG. 4 illustrates exemplary control channels included in the
control region of a subframe in a DL radio frame.
Referring to FIG. 4, a subframe includes 14 OFDM symbols. The first
one to three OFDM symbols of a subframe are used for a control
region and the other 13 to 11 OFDM symbols are used for a data
region according to a subframe configuration. In FIG. 4, reference
characters R1 to R4 denote RSs or pilot signals for antenna 0 to
antenna 3. RSs are allocated in a predetermined pattern in a
subframe irrespective of the control region and the data region. A
control channel is allocated to non-RS resources in the control
region and a traffic channel is also allocated to non-RS resources
in the data region. Control channels allocated to the control
region include a Physical Control Format Indicator Channel
(PCFICH), a Physical Hybrid-ARQ Indicator Channel (PHICH), a
Physical Downlink Control Channel (PDCCH), etc.
The PCFICH is a physical control format indicator channel carrying
information about the number of OFDM symbols used for PDCCHs in
each subframe. The PCFICH is located in the first OFDM symbol of a
subframe and configured with priority over the PHICH and the PDCCH.
The PCFICH is composed of 4 Resource Element Groups (REGs), each
REG being distributed to the control region based on a cell
Identity (ID). One REG includes 4 Resource Elements (REs). An RE is
a minimum physical resource defined by one subcarrier by one OFDM
symbol. The PCFICH indicates 1 to 3 or 2 to 4 according to a
bandwidth. The PCFICH is modulated in Quadrature Phase Shift Keying
(QPSK).
The PHICH is a physical Hybrid-Automatic Repeat and request (HARQ)
indicator channel carrying an HARQ ACK/NACK for an uplink
transmission. That is, the PHICH is a channel that delivers DL
ACK/NACK information for UL HARQ. The PHICH includes one REG and is
scrambled cell-specifically. An ACK/NACK is indicated in one bit
and modulated in Binary Phase Shift Keying (BPSK). The modulated
ACK/NACK is spread with a Spreading Factor (SF) of 2 or 4. A
plurality of PHICHs mapped to the same resources form a PHICH
group. The number of PHICHs multiplexed into a PHICH group is
determined according to the number of spreading codes. A PHICH
(group) is repeated three times to obtain a diversity gain in the
frequency domain and/or the time domain.
The PDCCH is a physical downlink control channel allocated to the
first n OFDM symbols of a subframe. Herein, n is 1 or a larger
integer indicated by the PCFICH. The PDCCH is composed of one or
more CCEs. The PDCCH carries resource allocation information about
transport channels, PCH and DL-SCH, an uplink scheduling grant, and
HARQ information to each UE or UE group. The PCH and the DL-SCH are
transmitted on a PDSCH. Therefore, an eNB and a UE transmit and
receive data usually on the PDSCH, except for specific control
information or specific service data.
Information indicating one or more UEs to receive PDSCH data and
information indicating how the UEs are supposed to receive and
decode the PDSCH data are delivered on a PDCCH. For example, on the
assumption that the Cyclic Redundancy Check (CRC) of a specific
PDCCH is masked by Radio Network Temporary Identity (RNTI) "A" and
information about data transmitted in radio resources (e.g. at a
frequency position) "B" based on transport format information (e.g.
a transport block size, a modulation scheme, coding information,
etc.) "C" is transmitted in a specific subframe, a UE within a cell
monitors, that is, blind-decodes a PDCCH using its RNTI information
in a search space. If one or more UEs have RNTI "A", these UEs
receive the PDCCH and receive a PDSCH indicated by "B" and "C"
based on information of the received PDCCH.
FIG. 5 illustrates a structure of a UL subframe in the LTE
system.
Referring to FIG. 5, a UL subframe may be divided into a control
region and a data region. A Physical Uplink Control Channel (PUCCH)
including Uplink Control Information (UCI) is allocated to the
control region and a Physical uplink Shared Channel (PUSCH)
including user data is allocated to the data region. The middle of
the subframe is allocated to the PUSCH, while both sides of the
data region in the frequency domain are allocated to the PUCCH.
Control information transmitted on the PUCCH may include an HARQ
ACK/NACK, a CQI representing a downlink channel state, an RI for
Multiple Input Multiple Output (MIMO), a Scheduling Request (SR)
requesting UL resource allocation. A PUCCH for one UE occupies one
Resource Block (RB) in each slot of a subframe. That is, the two
RBs allocated to the PUCCH frequency-hop over the slot boundary of
the subframe. Particularly, PUCCHs with m=0, m=1, and m=2 are
allocated to a subframe in FIG. 5.
Introduction of local areas to the LTE system in the future is
under, consideration. To reinforce service support per user, it is
expected that a new cell will be deployed based on the concept of
local area access.
FIG. 6 illustrates the concept of a small cell, which is expected
to be introduced to the LTE system.
Referring to FIG. 6, it is expected that a wider system bandwidth
is set in a frequency band having a higher center frequency, not in
a frequency band used in the legacy LTE system. Basic cell coverage
may be supported based on a control signal such as system
information in an existing cellular frequency band, whereas data
may be transmitted with maximum transmission efficiency in a wider
frequency band in a high-frequency small cell. Thus, the concept of
local area access targets at UEs with low-to-medium mobility in a
small area and small cells will be deployed, each having a distance
between a Base Station (BS) and a UE in units of 100 m, smaller
than existing cells having distances between a UE and a BS in units
of km.
Due to shorter distances between UEs and a BS and the use of a high
carrier frequency, these small cells may have the following channel
characteristics.
First of all, from the perspective of delay spread, as the distance
between a BS and a UE is shorter, a signal delay may be also
shorter. If the same OFDM-based frame as used in the LTE system is
adopted, a subcarrier spacing may be set to an extremely large
value, for example, a value larger than the existing subcarrier
spacing 15 kHz because a relatively wide frequency band is
allocated. A Doppler's frequency is higher in a high frequency band
than in a low frequency band, for the same UE speed. Therefore, a
coherence time may be extremely short. The coherence time is the
time over which a channel has static or uniform characteristics. A
coherent bandwidth is a bandwidth in which a channel has static or
uniform characteristics in time.
Only when a UE is synchronized with a BS, the UE may transmit a UL
signal and may be scheduled for data transmission. A main role of
an RACH is radio access in a transmission scheme that makes
asynchronous UEs orthogonal to one another or prevents coincident
accesses of the UEs as much as possible. The RACH will be described
in greater detail.
Regarding the usage and requirements of the RACH, a main function
of the RACH is UL initial access and short message transmission.
Although initial network access and short message transmission take
place on the RACH in a Wideband Code Division Multiple Access
(WCDMA) system, short message transmission is not performed through
the RACH in the LTE system. In addition, the RACH is transmitted
separately from an existing UL data channel in the LTE system,
compared to the WCDMA system. That is, while a UL data channel,
PUSCH has a symbol structure with a basic subcarrier spacing
.DELTA.f set to 15 kHz, the RACH has an SC-FDMA structure with a
subcarrier spacing .DELTA.f.sub.RA set to 1.25 kHz. Once UL
synchronization is acquired between a BS and a UE, the UE is
scheduled for orthogonal resource allocation and transmission in
the LTE system.
The structure of an RACH preamble will be described below. FIG. 7
illustrates a structure of an RACH preamble.
Referring to FIG. 7, an RACH preamble includes a Cyclic Prefix
(CP), a preamble sequence, and a Guard Time (GT). The CP is used to
compensate for a maximum channel delay spread and a Round Trip Time
(RTT), and the GT is used to compensate for the RTT. The CP is a
copy of the last part of an OFDM symbol, inserted into the CP
period of the preamble.
On the assumption that it has been synchronized with a BS, a UE
transmits an RACH preamble to the BS. If the UE is near to the BS,
the BS receives the RACH almost in alignment with a subframe
boundary. On the other hand, if the UE is remote from the BS, for
example, the UE is at a cell edge, the BS receives the RACH later
than a nearby UE's RACH due to a propagation delay. Because the BS
has knowledge of a preamble sequence transmitted by each UE, the BS
may perform a synchronization process based on the detected
position of the preamble transmitted by each UE.
Many sequences are available for an RACH preamble. For example, a
Zadoff-Chu (ZC) sequence based on auto-correlation and a
pseudorandom sequence based on cross-correlation are popular. In
general, the ZC sequence based on auto-correlation may be selected
in a low intra-cell interference environment and the pseudorandom
sequence based on cross-correlation may be selected in a high
intra-cell interference environment.
In the LTE system, 1) the intra-cell interference between different
preambles using the same time-frequency RACH resources should be
low; 2) since detection performance increases with the use of more
orthogonal preambles, the detection performance of a BS should be
increased by defining more orthogonal preambles for a smaller cell;
3) the detection complexity of the BS should be reduced; and 4) a
fast UE should also be supported. To meet the above requirements,
the LTE system uses a ZC sequence of length 839 expressed as
[Equation 1], for an RACH preamble.
.function..times..times..pi..times..times..function..ltoreq..ltoreq..time-
s..times..times. ##EQU00002##
However, if the intra-cell interference is high, a pseudorandom
sequence expressed as [Equation 2] may be used for an RACH
preamble. x.sub.1(n+31)=(x.sub.1(n+3)+x.sub.1(n))mod 2
x.sub.2(n+31)=(x.sub.2(n+3)+x.sub.2(n+2)+x.sub.2(n+1)+x.sub.2(n))mod
2 c(n)=(x.sub.1(n+N.sub.C)+x.sub.2(n+N.sub.C))mod 2 [Equation
2]
Now, a description will be given of a transmission bandwidth for an
RACH preamble. Two main factors taken into account in setting an
RACH bandwidth are diversity gain and restriction of UE
transmission power. Since a UE has limited power amplifier
performance relative to a BS, energy per resource unit is decreased
but frequency diversity is maximized by transmitting an RACH in a
wide frequency band. On the contrary, if an RACH preamble is
transmitted in a narrow frequency band, energy per resource unit is
increased but frequency diversity is minimized.
When an LTE RACH transmission bandwidth is determined actually,
1.08 MHz, 2.16 MHz, 4.5 MHz, and 50 MHz (having 6 RBs, 12 RBs, 25
RBs, and 50 RBs, respectively) are candidates. Since it is revealed
from a comparison of RACH non-detection probabilities that 6 RBs is
enough to satisfy a non-detection probability of 1%, 1.08 MHz is
determined as a final RACH transmission bandwidth.
The length of an RACH preamble sequence will be described now. To
determine the length T.sub.SEQ of an RACH preamble sequence,
conditions for the low and upper bounds of the sequence length
T.sub.SEQ and a subcarrier spacing should be satisfied.
The lower bound of the sequence length T.sub.SEQ should be larger
than the sum of the RTT and maximum channel delay spread of a
cell-edge UE within coverage in order to eliminate detection
ambiguity. That is, [Equation 3] should be satisfied.
.gtoreq..times..tau..times..times..times..times..times..times.
##EQU00003##
In [Equation 3], d.sub.long represents the service coverage and
.tau..sub.max represents the maximum channel delay spread. For
example, it is assumed that the largest cell has a radius of 100 km
and the maximum channel delay spread of the cell is 16.67 .mu.s in
the LTE system. It is also assumed that service coverage in a high
carrier frequency is 3 km and the maximum channel delay spread of
the high carrier frequency is 0.5 .mu.s. On these assumptions, the
following [Equation 4] and [Equation 5] are given.
.gtoreq..times..times..times..times..times..times..times..times..times..t-
imes..times..gtoreq..times..times..times..times..times..times..times..time-
s..times..times..times..times..times..times..times.
##EQU00004##
If the upper bound of the sequence length T.sub.SEQ is determined
in conformance to a general frame standard, the upper bound cannot
exceed a given Transmission Time Interval (TTI). If a subframe is 1
ms long as in the LTE system, the TTI is 1 ms. Herein, a maximum
sequence period is based on the assumption of service coverage in
which a UE is nearest to a BS and the maximum channel delay spread
is 0 .mu.s. Accordingly, condition #2 expressed as [Equation 6]
should be satisfied.
.ltoreq..times..times..times..times..times..times. ##EQU00005##
In [Equation 6], d.sub.short represents the service coverage in
which a UE is nearest to a BS. For example, d.sub.short is 14.4 km
in the LTE system and d.sub.short is 1 km in the high carrier
frequency. If the TTI is 222 ms in the high carrier frequency,
[Equation 7] and [Equation 8] are resulted.
.times..ltoreq..times..times..times..times..times..times..times..times..t-
imes..times..times..times..ltoreq..times..times..times..times..times..time-
s..times..times..times..times..times..times..times..times..times..times.
##EQU00006##
Finally, a requirement for the RACH subcarrier spacing
.DELTA.f.sub.RA will be described below.
If a sampling frequency N.sub.DFT being the reciprocal of the
sequence length T.sub.SEQ is in the relationship that
N.sub.DFT=f.sub.sT.sub.SEQ, maximum orthogonality is ensured
between UL subcarriers of an existing frame and RACH subcarriers.
Because the subcarrier spacing .DELTA.f of the existing frame
should be an integer multiple of the RACH subcarrier spacing
.DELTA.f.sub.RA, condition #3 given as [Equation 9] should be
satisfied.
.DELTA..times..times..DELTA..times..times..times..times..times..times.
##EQU00007##
In this case, the RACH subcarrier spacing .DELTA.f.sub.RA is
determined in the LTE system by the following equation.
.DELTA..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times. ##EQU00008##
Hereinbelow, an exemplary setting of an RACH preamble period for
RACH transmission in a high carrier frequency, satisfying condition
#1, condition #2, and condition #3 will be described. For a minimum
service coverage of 1 km and a maximum service coverage of 3 km, an
RTT is calculated and a maximum channel delay spread of 0.5 .mu.s
is considered.
TABLE-US-00001 TABLE 1 TTI- (GP + CP) RTT CP for RACH TTI Sequence
Coverage (GP) (RTT + 0.5 us) GP + CP candidate duration 1 km 6.6 us
7.1 us 13.7 us 222 us 208.3 us (short) 767 us 754.3 us 125 us 111.3
us 3 km 20.0 us 20.5 us 40.5 us 222 us 181.9 us (long) 767 us 726.9
us 125 us 84.9 us
If the service coverage is 3 km and the TTI is 222 .mu.s, an RACH
preamble period and a CP are calculated by [Table 2].
TABLE-US-00002 TABLE 2 k = .DELTA.f/.DELTA.f.sub.RA = 18 .ltoreq.
.left brkt-bot.T.sub.SEQ/T.sub.SYM.right brkt-bot. = .left
brkt-bot.181.9 us/(1/104.25 kHz).right brkt-bot. = 18
.DELTA.f.sub.RA = .DELTA.f/k = 104.24 kHz/18 = 5.7917 kHz T.sub.SEQ
= 1/.DELTA.f.sub.RA = 1.7266 us T.sub.CP = 20.5 us
As noted from [Table 3] below, condition #1, condition #2, and
condition #3 are all satisfied with the calculated sequence length
T.sub.SEQ=172.66 us
TABLE-US-00003 TABLE 3 Condition #1:
.times..times..gtoreq..times..times..times..times..times..ti-
mes..times. ##EQU00009## Condition #2:
.times..times..ltoreq..times..times..times..times..times..ti-
mes..times..times. ##EQU00010## Condition #3: .DELTA.f.sub.RA =
.DELTA.f/k = 104.24 kHz/18 = 5.7917 kHz
Therefore, a final RACH preamble may be configured as illustrated
in FIG. 8. FIG. 8 illustrates an exemplary setting of a CP and a
transmission period for an RACH preamble, for RACH preamble
transmission in a high carrier frequency. In the illustrated case
of FIG. 8, the service coverage is 3 km and the TTI is 222
.mu.s.
In another example, if the service coverage is 1 km and the TTI is
125 .mu.s, the values illustrated in [Table 4] may be
calculated.
TABLE-US-00004 TABLE 4 k = .DELTA.f/.DELTA.f.sub.RA = 12 .ltoreq.
.left brkt-bot.T.sub.SEQ/T.sub.SYM.right brkt-bot. = .left
brkt-bot.111.3 us/(1/120 kHz).right brkt-bot. = 13 .DELTA.f.sub.RA
= .DELTA.f/k = 120 kHz/12 = 10 kHz T.sub.SEQ = 1/.DELTA.f.sub.RA =
100 us T.sub.CP = 7.1 us
As noted from [Table 5] below, condition #1, condition #2, and
condition #3 are all satisfied with the calculated sequence length
T.sub.SEQ=100 us.
TABLE-US-00005 TABLE 5 Condition #1:
.times..times..gtoreq..times..times..times..times..times..t-
imes..times. ##EQU00011## Condition #2:
.times..times..ltoreq..times..times..times..times..times..t-
imes..times..times. ##EQU00012## Condition #3: .DELTA.f.sub.RA =
.DELTA.f/k = 120 kHz/12 = 10 kHz
Therefore, a fmal RACH preamble may be configured as illustrated in
FIG. 9. FIG. 9 illustrates another exemplary setting of a CP and a
transmission period for an RACH preamble, for RACH preamble
transmission in a high carrier frequency. In the illustrated case
of FIG. 9, the service coverage is 1 km and the TTI is 125
.mu.s.
To verify whether a target area of a sequence satisfying condition
#1, condition #2, and condition #3 is appropriately designed, link
budget parameters listed in [Table 6] may be used.
TABLE-US-00006 Parameter LTE value Higher Carrier Band Carrier
frequency(f.sub.c) 2 GMHz 30 GHz eNB antenna height(h.sub.b) 30
m/60 m 10 m(3 GPP 36.814 UMI) UE antenna height(h.sub.m) 1.5 m 1.5
m UE transmit Power 24 dBm(250 mW) 24 dBm (P.sub.max, EIRP) eNB
receiver Ant. Gain 14 dBi 14 dBi (G.sub.a) ReceivingNoise 5 dB 5 dB
Figure(N.sub.f) Thermal Noise -174 dBm/Hz -174 dBm/Hz
Density(N.sub.0) Required (E.sub.p/N.sub.0) 18 dB 18 dB Penetration
loss (PL) 0 dB 0 dB(outdoor) Log-normal fading 0 dB 0 dB(outdoor)
margin(LF) PL model(L(d))[dB] Okumura-Hata LMDS channel model
(Suburban areas) (Good, Bad) + Margin w.r.t height Target
coverage(d)[km] About 14 km 3 km(RTT = 19.8 us)
The target area of the sequence is finally verified by [Equation
11].
.times..function..times..times..times. ##EQU00013##
[Equation 11] is expressed as a function of distance d by which an
appropriate effective distance may be estimated. A verification
example regarding an LTE case and a high carrier frequency case
with a service coverage of 3 km and a TTI of 222 .mu.s will be
described below. It is assumed that the maximum channel delay
spread is 0.5 .mu.s.
A path loss function P.sub.RA(d) in [Equation 11] may be
represented as [Equation 12], in terms of dB.
P.sub.RA(d)=P.sub.max+G.sub.a-L(d)-LF-PL(dB) [Equation 12]
A substantial path loss is expressed as a function L(d) in
[Equation 12]. An Okumura-Hata model applies in designing an RACH
in the LTE system.
FIG. 10 is a graph illustrating RACH sequence length versus service
coverage in the LTE system. Particularly, a suburban situation of
the Okumura-Hata model is taken in FIG. 10. Referring to FIG. 10,
it is noted that if a BS height is 60 m at a point where the
sequence length T.sub.SEQ is 1 ms, the service coverage is about 14
km.
For verification in the high carrier frequency case, a Local
Multipoint Distribution Services (LMDS) model applies to the path
loss function L(d). FIG. 11 is a graph illustrating RACH sequence
length versus service coverage in the high carrier frequency
system. Referring to FIG. 11, it is noted that a service coverage
with a sequence length T.sub.SEQ of 111.3 .mu.s is appropriately 14
km, far larger than a target coverage of 3 km.
As described above, an RACH transmission period may vary with a
service coverage and a TTI. In addition, to maintain orthogonality
with an existing OFDM frame, an RACH subcarrier spacing should be
an integer multiple of an existing subcarrier spacing. This means
that the RACH subcarrier spacing gets shorter in the frequency
domain and an RACH OFDM symbol having a longer period than an
existing OFDM symbol period is set in the time domain. That is, an
RACH OFDM symbol is designed to be k times longer than an existing
OFDM symbol based on the relationship that
.DELTA.f.sub.RA=.DELTA.f/k.
In the present invention, an RACH sequence length is determined
based on the relationship between an OFDM symbol period and an RACH
OFDM symbol period.
FIG. 12 illustrates a relationship between an RACH subcarrier
spacing and an RACH sequence length.
Particularly, the length of an RACH sequence is determined for a
case where an RACH OFDM symbol period is longer than an existing
OFDM symbol period by N times in FIG. 12. As noted from the
frequency axis in the upper drawing of FIG. 12, the increase of the
period of an RACH OFDM symbol to an N multiple of the length of an
existing OFDM symbol means the decrease of the existing OFDM
subcarrier spacing .DELTA.f to 1/N. The RACH OFDM symbol period is
set substantially by .DELTA.f.sub.RA=.DELTA.f/k,
1.ltoreq.k.ltoreq.N as in [Equation 9].
That is, if the RACH symbol transmission period within a TTI is set
to be longer than the existing OFDM symbol period by N times, this
implies that the length of an actual RACH sequence transmitted by a
UE is equal to or smaller than an N multiple of the existing OFDM
symbol period. For example, if N=8, a container period T.sub.CON
during which an RACH may be transmitted is 8 times longer than the
existing OFDM symbol period T.sub.SYM.
If the RACH sequence length T.sub.SEQ is calculated by
T.sub.SEQ=1/.DELTA.f.sub.RA=2/.DELTA.f according to condition #1,
condition #2, and condition #3, k=2. Consequently, the actual RACH
sequence length T.sub.SEQ is reduced to 1/2, relative to the
container period T.sub.CON.
As described above, the present invention provides methods for
setting an RACH sequence length within a TTI longer than the
existing OFDM symbol period by N times.
<Embodiment 1>
If the actual length of an RACH sequence is smaller than N symbols
in an RACH transmission period N times longer than an existing OFDM
symbol period, the RA sequence may be repeated in the time domain,
for transmission.
For the convenience of description, if the RACH sequence is
designed to be k times longer than an existing OFDM symbol (i.e.
T.sub.SEQ=k T.sub.SYM) and the total RACH transmission period
T.sub.CON is equal to NT.sub.SYM, the repetition time of the RACH
sequence is determined by [Equation 13] because N.gtoreq.k.
.times..times..times..times..times..times. ##EQU00014##
FIG. 13 illustrates an exemplary setting of the length of an RACH
sequence that is transmitted repeatedly twice during a TTI
according to an embodiment of the present invention.
Referring to FIG. 13, if N=8 and k=4, the repetition time of an
RACH sequence is 2 and thus the length T.sub.SEQ of the RACH
sequence is doubled during a TTI. The repetition time is determined
based on a relationship between an RACH transmission period and an
RACH sequence length. If the repetition time is 1, the RACH
transmission time period is equal to the RACH sequence length
during a TTI in an extreme case.
<Embodiment 2>
If the actual length of an RACH sequence is smaller than N in an
RACH transmission period N times longer than an existing OFDM
symbol period, the RACH sequence may be repeated not in the time
domain but in the frequency domain.
As in Embodiment 1, if the RACH sequence is designed to be k times
longer than the existing OFDM symbol (i.e. T.sub.SEQ=k T.sub.SYM)
and the total RACH transmission period T.sub.CON is equal to
NT.sub.SYM, the repetition time of the actual RACH sequence length
is determined by [Equation 13] because N.gtoreq.k.
Compared to Embodiment 1 in which an RACH symbol is repeatedly
transmitted in the time domain, frequency-domain sequence
allocation is controlled such that one-period RACH symbols may be
transmitted during an RACH transmission period within a TTI in the
frequency domain. That is, if the length of an RACH sequence
x.sub.n, (0.ltoreq.n<N.sub.SEQ) is N.sub.SEQ, the total length
of the RACH sequence is increased to a multiple of a repetition
time, expressed as [Equation 14].
.times..times..ltoreq.<.ltoreq.<.times..times.
##EQU00015##
This method may be useful, when an RACH sequence length and symbol
period are determined so as to support various bandwidths.
FIG. 14 illustrates an exemplary allocation of an RACH sequence in
the frequency domain, for one-period transmission during a TTI
according to an embodiment of the present invention.
Referring to FIG. 14, if N=8 and k=4, the repetition time is 2. In
this case, while an RACH sequence of length T.sub.SEQ is
transmitted once during a TTI, the RACH sequence may be repeated in
the frequency domain.
This implies that the basic RACH subcarrier spacing .DELTA.f.sub.RA
is equal but a substantially occupied RACH subcarrier spacing is
2.DELTA.f.sub.RA in allocating one RACH sequence. In this case, the
substantial occupied RACH subcarrier spacing is given as the
following equation.
.times..times..DELTA..times..times..DELTA..times..times..times..times..ti-
mes..times. ##EQU00016##
As described above, the present invention provides a method for
designing an RACH along a time axis adaptively according to a
communication environment using a high carrier frequency.
Particularly, an operation scenario centering on a small cell is
feasible because path loss is large due to a high center frequency
of the high carrier frequency. However, the present invention is
not limited thereto.
FIG. 15 is a block diagram of a communication apparatus according
to an embodiment of the present invention.
Referring to FIG. 15, a communication apparatus 1500 includes a
processor 1510, a memory 1520, a Radio Frequency (RF) module 1530,
a display module 1540, and a User Interface (UI) module 1550.
The communication device 1500 is shown as having the configuration
illustrated in FIG. 15, for the convenience of description. Some
modules may be added to or omitted from the communication apparatus
1500. In addition, a module of the communication apparatus 1500 may
be divided into more modules. The processor 1510 is configured to
perform operations according to the embodiments of the present
invention described before with reference to the drawings.
Specifically, for detailed operations of the processor 1510, the
descriptions of FIGS. 1 to 14 may be referred to.
The memory 1520 is connected to the processor 1510 and stores an
Operating System (OS), applications, program codes, data, etc. The
RF module 1530, which is connected to the processor 1510,
upconverts a baseband signal to an RF signal or downconverts an RF
signal to a baseband signal. For this purpose, the RF module 1530
performs digital-to-analog conversion, amplification, filtering,
and frequency upconversion or performs these processes reversely.
The display module 1540 is connected to the processor 1510 and
displays various types of information. The display module 1540 may
be configured as, not limited to, a known component such as a
Liquid Crystal Display (LCD), a Light Emitting Diode (LED) display,
and an Organic Light Emitting Diode (OLED) display. The UI module
1550 is connected to the processor 1510 and may be configured with
a combination of known user interfaces such as a keypad, a touch
screen, etc.
The embodiments of the present invention described above are
combinations of elements and features of the present invention. The
elements or features may be considered selective unless otherwise
mentioned. Each element or feature may be practiced without being
combined with other elements or features. Further, an embodiment of
the present invention may be constructed by combining parts of the
elements and/or features. Operation orders described in embodiments
of the present invention may be rearranged. Some constructions of
any one embodiment may be included in another embodiment and may be
replaced with corresponding constructions of another embodiment. It
is obvious to those skilled in the art that claims that are not
explicitly cited in each other in the appended claims may be
presented in combination as an embodiment of the present invention
or included as a new claim by a subsequent amendment after the
application is filed.
The embodiments of the present invention may be achieved by various
means, for example, hardware, firmware, software, or a combination
thereof. In a hardware configuration, the methods according to
exemplary embodiments of the present invention may be achieved by
one or more Application Specific Integrated Circuits (ASICs),
Digital Signal Processors (DSPs), Digital Signal Processing Devices
(DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate
Arrays (FPGAs), processors, controllers, microcontrollers,
microprocessors, etc.
In a firmware or software configuration, an embodiment of the
present invention may be implemented in the form of a module, a
procedure, a function, etc. Software code may be stored in a memory
unit and executed by a processor. The memory unit is located at the
interior or exterior of the processor and may transmit and receive
data to and from the processor via various known means.
Those skilled in the art will appreciate that the present invention
may be carried out in other specific ways than those set forth
herein without departing from the spirit and essential
characteristics of the present invention. The above embodiments are
therefore to be construed in all aspects as illustrative and not
restrictive. The scope of the invention should be determined by the
appended claims and their legal equivalents, not by the above
description, and all changes coming within the meaning and
equivalency range of the appended claims are intended to be
embraced therein.
INDUSTRIAL APPLICABILITY
The method and apparatus for transmitting an RACH designed for
transmission in a high carrier frequency in a wireless
communication system have been described in the context of a 3GPP
LTE system. Besides, the present invention is applicable to many
other wireless communication systems.
* * * * *